Software Engineering for Students A Programming Approach

270 Chapter 19 ■ Testing
within a black box. Black box testing is also known as functional testing because it uses
only knowledge of the function of the program (not how it works).
Ideally, testing proceeds by writing down the test data and the expected outcome of
the test before testing takes place. This is called a test specification or schedule. Then
you run the program, input the data and examine the outputs for discrepancies between
the predicted outcome and the actual outcome. Test data should also check whether
exceptions are handled by the program in accordance with its specification.
Consider a program that decides whether a person can vote, depending on their age
(Figure 19.1). The minimum voting age is 18.
We know that we cannot realistically test this program with all possible values, but
instead we need some typical values. The approach to devising test data for black box
testing is to use equivalence partitioning. This means looking at the nature of the input
data to identify common features. Such a common feature is called a partition. In the
voting program, we recognize that the input data falls into two partitions:
1. the numbers less than 18
2. the numbers greater than or equal to 18
This can be diagrammed as follows:
Figure 19.1 The voting checker program
0 17 18 infinity
There are two partitions, one including the age range 0–17 and the other partition
with numbers 18 to infinity. We then take the step of asserting that every number within
a partition is equivalent to any other, for the purpose of testing this program. (Hence
the term equivalence partitioning.) So we argue that the number 12 is equivalent to
any other in the first partition and the number 21 is equivalent to any number in the